1. Field of the Invention
The present invention relates to a thin film forming apparatus which irradiates ion beam to a plasma of source gas while controlling the irradiation to a substrate so that the particles such as ions or radicals in the plasma are sufficiently energized to form a deposit of crystalline thin film on the substrate.
2. Description of the Related Art
A typical example of the thin film forming apparatus which is an application of the CVD apparatus is described in Unexamined Japanese Patent Publication (kokai) No. Hei 5-55194 (HO1L 21/31). A high-frequency plasma of source gas is formed within a film forming chamber while, at the same time, ion beams from an ion source are directed into the film forming chamber to irradiate the high-frequency plasma and the surface of a substrate. Consequently, a uniform and homogeneous deposit of thin film is formed on the substrate at low temperature due to the migration effect of the excited species in the plasma from the beam irradiation and by the control of film quality.
The thin film forming apparatus described above is remarkably effective to form thin-film transistors in liquid-crystal display panels of TFT type and crystalline, silicon thin films in solar cells.
FIG. 6 is a schematic view of the conventional thin film forming apparatus described in the above Japanese Patent Publication. A vacuum chamber 1 is composed of a film forming chamber 2 as the lower part and an ion source 3 as the upper part. The bottom of the film forming chamber 3 is connected to a vacuum for constant evacuation.
A substrate holder 4 is provided in the lower part of the film forming chamber 2 to support a slab or plate of substrate 5 horizontally.
In the film forming chamber 2, a plasma (high-frequency plasma) 6 of source gas (reactant gas) is generated above the film-forming surface of the substrate 5 by discharging microwaves.
Ion beams 8 from the ion source 3 are drawn out from the ion source 3 via a drawing-out electrode 7 into the film forming chamber 2 underneath and pass through the plasma 6 to vertically irradiate the film-forming surface of the substrate 5.
In the drawing, reference numeral 9 designates a source gas inlet; 10, a microwave inlet; and 11, a field magnet assembly.
By the vertical irradiation of the ion beams 8, the particles such as ions or radicals in the plasma 6 are excited to a higher energy. At the same time, the substrate 5 is irradiated with the ion beams 8, whereupon the migration effect and the reaction on the surface of the substrate 5 are accelerated to form a crystalline thin film such as one of silicon on the substrate 5.
In the conventional apparatus shown in FIG. 6, the ion beams 8 are vertically irradiated to the surface of the substrate 5, i.e., in the direction of thin film deposition. Hence, particularly in the case of drawing-out the ion beams 8 at high voltage from the ion source 3, the ion beams 8 are bombarded against the substrate 5 with a great energy. Consequently, the substrate 5 may be damaged by the irradiating ion beams 8, thereby making the formation of a satisfactory thin film impossible.